Lamp ballasts usually include a converter having output terminals for connecting a fluorescent lamp and input terminals for applying an input voltage. The input voltage is a direct current (DC) voltage that is provided, for example, from a line voltage by a transformer stage. The converter generates from this DC voltage an AC voltage for operating the lamp, with the frequency of this alternating current (AC) voltage determining the operating state of the converter and thus of the lamp.
It is known to preheat the lamp before it is turned on for the first time or before it is turned on after a long off time, say several minutes. To this end, an AC voltage having a frequency higher than a later operating frequency of the lamp is generated by the converter. Once a specified preheat time has been reached, the lamp can then be ignited by lowering the frequency of the AC voltage to an ignition frequency. After the lamp has been ignited, the AC voltage is provided at the operating frequency. This operating frequency lies in the range of the ignition frequency.
If a fluorescent lamp turns off, for example because of an outage of the power supply, the lamp may be re-ignited immediately without a preheat phase once the power supply is restored, provided that the duration of the power outage is shorter than a maximum permissible waiting time, which is for example in the range of a second or a few seconds. Such brief outages of the power supply can occur for example in public buildings that have an emergency power supply and in which, upon an outage of a main power supply, an emergency supply is available—at least for selected circuits—within an interval of usually less than one second.